It is well known to prepare waste paper for re-use in paper-making processes by subjecting it to de-inking. The waste paper is normally pulped with water in the presence of de-inking chemicals and then the pulp is subjected to a de-inking stage in which ink is separated from the pulp. The de-inking stage may involve for instance flotation and/or washing. Flotation generally takes place in a flotation cell, in which a foam is formed at the surface of the pulp. This foam is removed, since it contains large amounts of the ink. However, this foam may contain some desirable materials, for instance fibres and cellulosic fines. Significant amounts of fibres and cellulosic fines are also often carried through in the waste water which has been used for washing the deinked pulp (backwater).
In general it is well known simply to dispose of the foam from the flotation stage. Backwater is often treated so as to clarify it. That is, solid materials are separated from the backwater to form a sludge. The clarified water can then be recycled to the de-inking process and the sludge sent for disposal. Clarification can be by for instance sedimentation, filtration or dissolved air flotation. All of these processes are designed to remove from the backwater substantially all suspended solids in a non-selective manner.
Thus in such systems any fibres and cellulosic fines which have been carried into the foam or backwater are lost.
It is known to screen backwater before it is subjected to clarification treatment so as to remove long cellulosic fibres. These can then be recycled to the de-inking or paper-making process. The filtrate is then treated as normal to remove all remaining suspended solids. This is often done by dissolved air flotation. Again, substantially all suspended solids are removed in a non-selective manner. It is also known to include flocculating or coagulating chemicals at this stage. These induce agglomeration of the suspended solids into flocs which can then be floated and removed. Systems of this general type are described by Krofta and Wang, in "Total Closing of Paper Mills with Reclamation and De-inking Installations", 43rd Purdue Industrial Waste Conference Proceedings, 10-12 May 1988, pp. 673 to 687 (published 1989). These systems aim only to recover long fibres from backwater. After these have been recovered, the aim of the described system is to clarify and recycle the backwater.
It is also known to clarify paper-making white waters in general by removing fibres using standard solid-liquid separation methods.
In all of the above systems, even those which involve recovery of long fibres, valuable materials are lost, in particular cellulosic fines. Not only are these useful materials lost, but they form part of the foam or sludge which must be disposed of and contribute towards the significant disposal costs (either by landfill or burning) which are incurred by de-inking plants.
Dorica and Simandl, in "Separation of Fibre and Ash in De-inking Effluents: A Case Study", Tappi Journal, Vol. 78, No. 5, pp 109 to 116, May 1995, have attempted to separate useful cellulosic materials from other materials such as clay present in de-inking backwaters. They aim to do this by using modified mechanical systems which include dispersed air flotation and screening. The flotation step is designed to float the fibres to form an upper fraction and to retain clay in the lower fraction. However, from the results given it appears that significant amounts of the fibre content of the suspension treated are lost in the lower, "clay" fraction. Dorica and Simandl also describe various two-stage processes. Those described however involve either two screening steps or a flotation step followed by screening of the floated materials.
The systems described all rely on solely mechanical methods and do not give an efficient separation of cellulosic and hydrophobic materials.
It would be desirable to be able to reduce the costs of waste disposal by re-use of valuable materials, by separating these in an efficient and convenient manner from materials which are not required for re-use.
It would also be desirable to avoid as far as possible the loss of suspended cellulosic materials from waste waters, especially those produced in the course of de-inking processes, and to recover these for re-use in an efficient manner.